Many industrial applications exist where materials are exposed to severe environmental conditions of heat, wear and corrosion. Spray coating processes utilizing powder coating materials offer high quality protection in some of these applications. A common method of spray coating is the detonation gun process. This process uses kinetic energy from the detonation of combustible mixtures of gases to deposit powdered coating materials on workpieces.
Typical coating materials used in conjunction with detonation gun in the spray coating process include powder forms of metals, metal-ceramic, ceramic, erosion resistant, thermal protection, electrically insulating, electrically conductive, and other coating materials. In addition powder forms of other materials can be utilized in conjunction with the detonation gun process for parts cleaning, hole drilling, making powders, and other conceivable applications.
A typical detonation gun functions in the following manner. A certain amount of a combustible gas mixture, oxygen and acetylene for example, is fed into a tubular combustion chamber have a closed end and an open end where it is subsequently ignited by a spark plug. The ignition of the gas brings about detonation and the formation of a shock wave. The shock wave travels down the combustion chamber to the open end which is attached to a tubular barrel. A suitable coating powder is typically injected into the barrel in front of the propagating shock wave and is subsequently carried out the open end of the barrel and deposited onto a substrate positioned in front of the barrel. The impact of the powder onto the substrate produces a high density coating with good adhesive characteristics. The process is repeated in a rapid fashion until the workpiece is coated to satisfaction. Between successive ignitions an inert gas, such as nitrogen, may be fed into the combustion chamber after the ignition to halt combustion and prevent backfire into the fuel and oxygen supply and to pure the barrel of combustion products.
The mechanics of detonation are key to the operation of the detonation gun. Detonation produces shock waves that travel at supersonic velocities, as high as 4000 m/s, and elevated temperatures, as high as 3137.degree. C. Detonation in the detonation gun is controlled by the type of fuel used, such as propane, acetylene, butane, etc., the fuel and oxygen mixture ratio, the initial pressure of the gases in the combustion chamber, and the geometry of the combustion chamber. After ignition of the fuel and oxygen mixture deflagration produces an initial detonation wave front that increases the temperature and pressure within the combustion chamber which in turn propagates ignition of the combustible mixture throughout the combustion chamber. Given the correct combination of parameters, the detonation continues to propagate until all available fuel and oxygen is consumed. The detonation front moves toward the open end of the combustion chamber and into the barrel. It is of particular importance that the combustion chamber be of sufficient length, for the specific detonable mixture in use, to complete the transition from deflagration to detonation before entering the barrel or the detonation wave front may not be sustained within the barrel. It is also important in the operation of a detonation gun to produce as strong a shock wave as possible and direct it to the barrel as efficiently as possible so that a large amount of the kinetic energy of the detonation wave goes directly to carrying the powder out of the barrel and onto the substrate.
At a fixed moment in time the detonation wave front is made up of a system of individual stationary detonation cells. The behavior of detonation at the cell level is an important attribute in the control and operation of a typical detonation gun. The detonation cell is a multidimensional structure which includes both the detonation wave front and transverse detonation waves moving perpendicular to the detonation front. The frontal surface of a detonation cell consists of convex shaped mach wave. Behind the mach wave is a reaction zone where the chemical reactions take place that lead to detonation. At the edge of the cell transverse shock waves form at substantially right angles to the frontal surface of the detonation cell. The transverse waves have acoustic tails that extend from the aft edges of the transverse waves and define the aft edge of the detonation cell. The transverse waves move from cell to cell and reflect off of each other and off of any limiting structure such as the combustion chamber wall. Once detonation has been initiated the reaction continues in a fairly stable fashion. However, the detonation wave front structure can be negatively influenced by collisions with reflecting transverse waves and reflecting refracted waves from the detonation front while moving through the combustion chamber. These collisions diminish the intensity of the detonation cells and therefor lessen the amount of kinetic energy available to be transferred to the coating powder. This reduction in energy transferred to the coating powders translates into a reduction of the coatings produced in terms of density and adherence with the substrate. The residuum of detonation wave front moves from the combustion chamber into the barrel and out onto the workpiece.
The size of the detonation cell is another important attribute in the control and operation of a detonation gun. Cell size is a function of the molecular nature of the fuel, the initial pressure within the combustion chamber, and the fuel/oxygen ratio. The particular cell size for certain conditions can be determined experimentally. The width of a cell, Sc, is measured along the wave front between successive transverse waves. The length of a cell, Lc, is the perpendicular distance from a line tangent to the wave front measured to the intersection point of the acoustic tails from adjacent transverse waves. The typical ratio of cell width to cell length is Sc=0.6 Lc for the detonable gases under consideration. The physical parameters of a particular typical detonation gun, such as the geometry and operating pressures, are determined by the cell size of a particular fuel and oxygen mixture.
The operating pressure within the combustion chamber is influenced by the behavior of the detonation cells. Prior to ignition the pressure within the combustion chamber is controlled by the fuel and oxygen supply pressures and the geometry of the combustion chamber. After ignition of the mixture the pressure within the combustion chamber increases and reaches a maximum when detonation occurs. As the detonation wave travels down the barrel and reaches the open end of the barrel a peak rarefaction pressure is measured within the combustion chamber. A positive pressure peak is then subsequently measured within the combustion chamber due to the presence of reflected waves from the detonation wave front.
In a typical detonation gun the coating powder, such as AMPERIT.TM. is fed either directly into the barrel directly or into the combustion chamber and then carried into the barrel by inert gases ahead of the detonation wave. For example, a certain powder feeder utilizes a continuous supply of air or inert gas to carry the powder fed from a continuous source through a valve arrangement and finally into the gun. The operation of the valve is coordinated with the firing of the spark plug so that the powder and carrying gases are in position along the barrel to be properly effected by the detonation wave. Typically the valves are opened by mechanical means such as a cam and tappets or a solenoid. The disadvantage of these mechanisms is that they often limit the frequency at which the gun can fire because the valve must be opened far enough and long enough to permit the passage of the proper amount of powder through the valve. These mechanisms also pose reliability problems in that they have rapidly moving pieces and transport powders that tend to be abrasive in nature leading to gun life cycle and maintenance concerns. In addition, valves pose safety concerns in that a valve that leaks, sticks open or breaks gives an alternate and potentially harmful path for the detonation wave front to escape.
The rate at which a detonation gun deposits the coating powder on the workpiece is an important economic parameter in industrial applications. The deposition rate is controlled, and at times limited, by a variety of factors such as the type of fuel, the fuel supply system, the geometries of the combustion chamber and barrel, the powder feeder system, and the purging of the system between successive ignitions. Deposition rate is expressed as the ratio between the spray rate and spray spot square. The spray rate is stated in terms of the mass of coating powder utilized per unit time, typically Kg/hr, and typically ranges from 1 to 6 Kg/hr. Spray rate is obviously influenced to great extend by the rate at which the spark plug is ignited. In a typical detonation gun the spark plug is ignited at the maximum rate of 6 to 10 times per second. The spray spot square is the area coated by a single ignition of the gun and is roughly equal to the area of the barrel and is typically expressed as mm.sup.2. A typical industrial detonation gun has a deposition rate of about 0.001 to 0.02 Kg/mm.sup.2 -hr.
In the typical detonation gun the combustible fuels and oxygen are supplied in gas form either into a mixing chamber or directly into the combustion chamber itself through a series of valves. The combustible gases are supplied under pressure of about 1 to 3 Mpa from a continuous source to the valve system before being issued into the gun. The opening of the valve system is synchronized to properly proportion the gases and to prevent backfire. As discussed previously a valve system as employed in a typical detonation gun raises serious concerns about rate, reliability and safety.
An important characteristic affecting the quality of the coatings produced by the detonation gun is the supersonic velocities at which the shock waves travel. The shock waves carry the coating powders at such velocities and, therefore, the coatings that are produced achieve higher densities and better adhesive qualities than other spray coating methods. The velocity of the coating powder as it exits the barrel is influenced by, among other things, the type of fuel used, and the geometries of the combustion chamber and barrel. Typical detonation wave velocities for detonable gas mixtures lie between 1200 m/sec and 4000 m/sec with H.sub.2 --O.sub.2 at 2830 m/sec and CH.sub.4 --O.sub.2 at 2500 m/sec. The maximum achievable velocity in present detonation gun configurations is approximately 3000 m/sec.
The temperatures surrounding the operation of a detonation gun is yet another important characteristic affecting the quality of the coatings produced and concerning its use as an industrial coating apparatus. Typical adiabatic flame temperatures for detonable gas mixtures of concern range from 1947.degree. C. to 3137.degree. C. with H.sub.2 --O.sub.2 at 2807.degree. C. and CH.sub.4 --O.sub.2 at 2757.degree. C. It is often desirable to melt the coating powders before depositing them on the substrate and given the correct parameters these temperatures are high enough to melt certain powder coating materials. The temperature imparted to the powders is in part controlled by barrel geometry and in part controlled by active cooling of the barrel. These temperatures are high enough to melt most substrate materials, however, the discontinuous nature of the combustion within a detonation gun prevents the substrate from being adversely affected.
The use of non-combustible gases in the operation of a detonation gun also effects the quality of the coatings produced. There are three common uses of non-combustible gases in detonation gun operations: 1. As purging gases; 2. As powder carrier gases; and 3. As a control on the detonation process. Purging gases typically are inert gases and are used primarily to purge the combustion chamber between successive firings of the spark plug to arrest the combustion process. This is important in the typical detonation gun because the combustion chamber must be filled between successive firings of the spark plug with new amounts of combustible fuel and oxygen mixture through a series of valves. If combustion continued in the combustion chamber while the valves are opened it is possible that the combustion would continue into the fuel and oxygen supply and cause an explosion. One of the problems with using purging gases is that they mix with the combustible gases and lower the overall kinetic energy of the detonation because the inert gases are by their very nature non-combustible. Therefore the kinetic energy available for transferring to the coating powders is lessened and coating density and adhesion will be adversely affected. In addition, the purging gases mix with the coating powder and slightly alter the final composition of the coatings produced. Powder carrier gases, frequently compressed air, are typically used to transfer the coating powders from a reservoir to the barrel of the detonation gun in front of the detonation wave front. These gases also lessen the kinetic energy available for transfer to the coating powders because they lower the temperature and velocity of the detonation wave front. The effect on coating quality is evidenced by a lower density coating and poorer adhesion with the substrate. As a control on the detonation process, inert gases are also mixed with the detonable gases. These are typically used in small amounts to control the temperature, velocity and chemical environment of the combustible products.